RF can flow on both sides of the aircrafts skin and it appears you have a
typical common mode issue. If you would be a bit more specific about the
frequency, since VHF/UHF covers from 30 mHz to 3 gHz, a solution can be
suggested.
Carl
----- Original Message -----
From: <qrv@kd4e.com>
To: <amps@contesting.com>
Sent: Thursday, April 20, 2017 7:14 PM
Subject: Re: [Amps] RF in Vehicles
He was speaking in shorthand.
Of course the radio is the "source" of the RF.
Presuming that the coax is properly connected to the radio, the
radio is properly grounded, and the antenna is resonant then the
problem would seem to be that something is coupling the RF from
the antenna into the wiring to the Supervisory Control Panel ...
or, the RF is somehow getting back though the power supply line.
If the antenna is non-resonant, for some reason, then the coax
can unintentionally become part of the antenna system ...
1KW at VHF/UHF is a ton of RF anywhere - but in the closed system
of an aluminum container - wow. You don't have to misplace much
of that power to cause mischief.
Given your altitude I wonder about the need for so much RF power,
but that's your business.
Just one non-engineer Ham's thoughts ...
73, DavidC KD4E
So I have this airplane. Not a Cessna 150, but not a B-52 either. Jet
with with about 30 KW available as the power source. The transmitter
consists of a phased combiner with four PA stages adding up to well over
a KW in the VHF/UHF range. The antenna is a basic monopole that uses the
complete mounting base as a ground plane. This arrangement is mounted on
the aircraft centerline. The aircraft itself makes a fairly good Faraday
Shield in that it has a solid surface of aluminum that is overlapped and
bonded to the internal structure. Never-the-less, there are several
wiring bundles that connect to this transmitter assembly for command and
control that are fed inside of the aircraft and thus by-pass the "Faraday
Shield".
When the transmitter is turned on, one of the internal generators in the
aircraft immediately turns off. The generator is a 3 phase 115 VAC 400
Hz model that uses an internal permanant magnetic generator at 40 volts
AC 800 Hz, that is in turn regulated to control the field of the
generator. This is controlled by a Supervisory Control Panel that
controls the field and also monitors for over/under voltage and
frequency.
Clearly the Supervisory Control Panel is being impacted by RFI thus
shutting the generator down.
The comment was: "This makes no sense. Unless something is very wrong
with the radio, there should be no RF on power or control wiring. From
an EMC point of view, the ANTENNA and it's counterpoise (metallic parts
of the vehicle) are the source of RF, NOT the radio. Clearly, whoever at
Toyota wrote this doesn't have a clue."
Ok, so what's wrong with this airplane?
Mark Bitterlich
WA3JPY
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